On Tue, Jun 28, 2016 at 10:38:28AM -0700, Andrey Vagin wrote:
> The problem is that a pathname can contain absolute symlinks and now
> they are resolved relative to the current root.
>
> If we want to open a file in another mount namespaces and we have a file
> descriptor to its root directory, we probably want to resolve pathname
> in the target mount namespace. For this we add this new flag.
>
> If LOOKUP_DFD_ROOT is set, path_init() initializes nd->root and nd->path
> to the same value.
>
> Signed-off-by: Andrey Vagin <avagin@xxxxxxxxxx>
Hi, Andrey,
Seems like a useful feature. Make sure to cc linux-api@xxxxxxxxxxxxxxx
for new userspace interfaces. One comment on the implementation below.
> ---
> fs/namei.c | 12 +++++++++++-
> include/linux/namei.h | 2 ++
> 2 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 70580ab..5f08b69 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2148,7 +2148,7 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
> nd->path.dentry = NULL;
>
> nd->m_seq = read_seqbegin(&mount_lock);
> - if (*s == '/') {
> + if (*s == '/' && !(flags & LOOKUP_DFD_ROOT)) {
> if (flags & LOOKUP_RCU)
> rcu_read_lock();
> set_root(nd);
> @@ -2174,6 +2174,11 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
> get_fs_pwd(current->fs, &nd->path);
> nd->inode = nd->path.dentry->d_inode;
> }
> + if (flags & LOOKUP_DFD_ROOT) {
> + nd->root = nd->path;
> + if (!(flags & LOOKUP_RCU))
> + path_get(&nd->root);
You're not initializing nd->root_seq here. That means that if we end up
going through unlazy_walk(), we're going to call legitimize_path() (and
thus read_seqcount_retry()) with stack garbage, get a spurious ECHILD,
and do an unnecessary restart of the path lookup instead of dropping
into ref-walk mode.
> + }
> return s;
> } else {
> /* Caller must check execute permissions on the starting path component */
> @@ -2202,6 +2207,11 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
> nd->inode = nd->path.dentry->d_inode;
> }
> fdput(f);
> + if (flags & LOOKUP_DFD_ROOT) {
> + nd->root = nd->path;
> + if (!(flags & LOOKUP_RCU))
> + path_get(&nd->root);
> + }
Same here.
The following should do the trick:
diff --git a/fs/namei.c b/fs/namei.c
index 9958b605e822..101d1fb8d3cb 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2176,7 +2176,9 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
}
if (flags & LOOKUP_DFD_ROOT) {
nd->root = nd->path;
- if (!(flags & LOOKUP_RCU))
+ if (flags & LOOKUP_RCU)
+ nd->root_seq = nd->seq;
+ else
path_get(&nd->root);
}
return s;
@@ -2209,7 +2211,9 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
fdput(f);
if (flags & LOOKUP_DFD_ROOT) {
nd->root = nd->path;
- if (!(flags & LOOKUP_RCU))
+ if (flags & LOOKUP_RCU)
+ nd->root_seq = nd->seq;
+ else
path_get(&nd->root);
}
return s;
--
Omar
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